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Storm frenzy is not an anomaly, but a phase

Atlantic current may be creating a hurricane hatchery. And the cycle may last 20 years.

By BILL COATS
Published September 13, 2005

Back in 1995, surface waters in the north Atlantic Ocean warmed up a smidgen.

The change was less than a degree, but it marked the first time in a quarter-century that waters were consistently warmer than average.

Storm experts warned of more hurricanes.

But nobody grasped the sweeping change that Mother Nature had signaled.

The 10 years since then have been the stormiest decade in the recorded history of the Atlantic basin. Mitch tore up Central America. Four hurricanes hammered Florida last year. Katrina decimated the Gulf Coast.

Now climatologists say frenzied hurricane seasons will be a fact of life for the next 10 to 20 years, part of a lengthy cycle of stormy eras followed by calmer ones.

The engine driving these cycles is called the Atlantic Multidecadal Oscillation, or AMO. Scientists say it has triggered drought in the western United States while spawning hurricanes in the Atlantic.

At a time when some are theorizing that global warming may be the reason for more intense hurricane seasons, climatologists say the AMO is the real culprit.

"The consensus among hurricane researchers and forecasters is that the hurricane landfalls of 2004 resulted from the AMO, a natural cycle of hurricane activity, combined with a lapse in the incredibly good fortune of the previous 35 years," Hugh Willoughby, a hurricane researcher at Miami's Florida International University, wrote in an essay last fall.

"The effect of global warming was at most second order," he wrote, "and probably not present at all."

Mayhem, questions

Today's climate researchers owe a debt to mariners of the late 1800s.

"They would lower buckets into the water and measure the temperatures," said Thomas Delworth, a physical scientist at a National Oceanic and Atmospheric Administration laboratory in New Jersey.

Steamship engines used ocean water to cool steam in their condensers.

The colder the ocean, the stronger the engines ran. Chief engineers kept meticulous temperature logs, helping them predict their top speeds.

Now those logs are helping scientists unravel the cycles of the Atlantic.

After studying temperature records dating to 1854, two University of Illinois researchers reported in a 1994 edition of Nature that air and surface-water temperatures in the north Atlantic were cyclically rising, then falling, over 65 to 70 years.

William Gray, the renowned hurricane forecaster from Colorado State University, also was studying the Atlantic. The warmer surface water in 1995 prompted Gray to predict an unusually stormy hurricane season.

His warning proved too tame.

Eleven hurricanes and eight tropical storms erupted in 1995, the highest tally since 1933. Hurricane Opal, after strengthening into a major hurricane the night before it struck Pensacola Beach, inflicted $3-billion in damage.

Such mayhem prompted Gray to question his statistical analyses. He concluded the 1995 ocean warming had rendered them unreliable.

Gray began giving top emphasis to water temperatures in the Atlantic.

By 1997 his forecasts began warning of "a new era" of hurricanes.

A flurry of studies ensued.

In one, Steve Gray, an Arizona-based research associate with the U.S. Geological Survey, led a team that tracked the weather cycles backward by studying ancient tree rings from Europe and the southern United States. Healthy weather produced wide tree rings. Drought or other trauma caused narrow rings.

The climate cycles kept repeating.

"It's been working in the same way for at least five centuries or so," said Gray, whose study was published last year.

How far back might the cycles extend?

"I'll go out on a limb and say at least one or two millennia," he replied.

Climatologists had long known that ocean temperatures influence weather, earlier reinforced by the Pacific Ocean's El Nino phenomenon.

But discoveries about the AMO in the mid-1990s helped explain why certain types of weather - storms, drought and rainfall - unfold in long patterns.

Researchers learned that AMO cycles depend on how fast the surface waters of the Atlantic flow north past Greenland, chill in the Arctic wind, then sink and head back south. It's like a liquid conveyor belt.

If the conveyor belt slows, surface waters have more time to cool as they journey north. If the belt speeds up, the water stays warmer farther north. This is the AMO's warm phase, the hurricane hatchery.

Why this flow speeds up and slows down is largely a mystery. NOAA's Delworth thinks the key influence is the rhythm of the Arctic winds.

Salinity matters, too. When evaporation makes the water saltier, it is denser and quicker to sink.

Lucky Florida

Records show the AMO was cool from 1900-1925, warm from 1926-1969, cool from 1970-1994 and warm since 1995.

Climatologists look at those dates and realize a generation of Americans is virtually blind to the true threat of hurricanes, having never experienced a major hurricane firsthand, at least until last year's four Florida hurricanes.

"During the time when so few hurricanes hit North America, we as a society framed decisions about land use, construction standards and other aspects of our lives around the shores of the Atlantic Ocean and Gulf of Mexico," wrote FIU's Willoughby last fall. "Built into those plans was the unstated assumption that hurricanes would continue to stay away from our shores as they had for the last third of a century."

Another expert said the hurricane seasons of the 1940s, in the heart of the last AMO warm phase, would stun today's Floridians.

"Imagine variations of 2004 occurring every year for 10 years," said Roger Pielke Jr., a University of Colorado professor who studies risk and has written a book about hurricanes.

Moreover, some researchers say records for the 1940s and earlier may undercount that era's storms because reconnaissance flights and hovering satellites still were in their infancies.

"We don't know what was going on out in the middle of the ocean," Willoughby said.

But they know Florida has been extraordinarily lucky.

"The last major storm to come through Florida, before Hurricane Andrew hit in 1992, was Hurricane Betsy in 1965, which went through the Keys," William Gray, the hurricane forecaster, told Discover Magazine in its September issue.

"Eight of the last 10 years have been very active," he said. "In fact, we've never had as much activity on the records, going back to about 1870 or so, as in the past 10 years - and yet we went from 1992 until last year with no hurricanes coming through Florida."

Troubling trend

As the 2005 hurricane season continues to rage, NOAA reports that half the nation now lives on 17 percent of the land: the coasts.

"A major hurricane - a Category 3, 4 or 5 - can't come ashore anywhere in the United States without causing a major disaster," Pielke said.

Pielke has calculated that a 1926 hurricane that ravaged Miami would create $110-billion in damage today. The 1921 hurricane that created a 15-foot tide in Oldsmar would cause $6-billion in damage, Pielke estimated.

Meanwhile, the Atlantic waters have continued to warm since the AMO switched to its warm phase in 1995, said Phil Klotzbach, research associate of forecaster Gray.

"This is the second warmest year we've ever seen over the tropical Atlantic," Klotzbach said. "It's pretty toasty out there."